1. Field of the Invention
This invention relates to an improvement in a process for the molybdenum-catalyzed reaction of an olefinically unsaturated compound with an organic hydroperoxide. More particularly, this invention relates to an improvement in a process for the molybdenum-catalyzed reaction of an ethylenically unsaturated compound such as an ethylenically unsaturated alkyl, cycloalkyl, aryl, etc., compound with an alkyl, cycloalkyl, aryl, etc., hydroperoxide wherein the ethylenically unsaturated compound is converted to the corresponding epoxide. Still more particularly, this invention relates to an improvement in the process for resolving the reaction mixture that is formed in preparing propylene oxide and tertiary butyl alcohol by reacting propylene with tertiary butyl hydroperoxide in solution in tertiary butyl alcohol in the presence of a soluble molybdenum catalyst. Even more particularly, this invention is directed to substantially removing molybdenum from the reaction mixture.
Molybdenum compounds are somewhat toxic to livestock and, therefore, solutions containing molybdenum must be handled with care. Also, the presence of molybdenum in liquid by-products presents a disposal problem because of the limited toxicity of molybdenum to livestock.
The epoxidation reaction mixture that is formed when an ethylenically unsaturated compound such as propylene is reacted with an organic hydroperoxide such as tertiary butyl hydroperoxide in the presence of a soluble molybdenum epoxidation catalyst will normally comprise, for example, unreacted propylene, propylene oxide, tertiary butyl alcohol, unreacted tertiary butyl hydroperoxide, the soluble molybdenum catalyst and impurities, including C.sub.1 to C.sub.4 lower aliphatic carboxylic acids. The reaction mixture is usually separated by distillation into a plurality of fractions including recycle propylene fraction, a propylene oxide product fraction, a tertiary butyl alcohol product fraction and a heavy liquid distillation fraction containing tertiary butyl alcohol, unreacted tertiary butyl hydroperoxide and impurities, including substantially all of the dissolved molybdenum catalyst and a portion of the lower aliphatic carboxylic acid impurities.
In accordance with the present invention, the heavy liquid distillation fraction is charged to a molybdenum separation zone where it is treated with an amount of ammonia sufficient to precipitate the bulk of molybdenum. The filtrate, which has a greatly reduced electrical resistance as compared to the heavy liquid distillation fraction, is placed in an electrolytic cell and a direct electric current is then passed through the ammonated solution in order to collect a significant amount of the molybdenum in solid form at the negative electrode. In accordance with a preferred embodiment, the heavy liquid distillation fraction is first charged to a molybdenum precipitation reactor where it is brought into liquid phase contact with an excess of ammonia (based on the molybdenum content). Preferably, the heavy liquid distillation fraction is saturated with ammonia. As a consequence, the ammonia will react with substantially all of the molybdenum to form a molybdate precipitate that can be separated from the thus-treated heavy liquid distillation fraction in a precipitate separation zone. The separate precipitate will comprise a concentrated solid molybdenum product that may be processed for the recovery of molybdenum. The precipitate will contain a substantial amount, but not all of the molybdenum present in the initial reaction mixture and the liquid portion of the heavy liquid distillation fraction remaining after removal of the molybdenum precipitate will still contain a residual amount of molybdenum. In accordance the present invention, the liquid portion, comprising an ammonated solution of the residually dissolved molybdenum, is charged, as the electrolyte, to an electrolytic cell (a molybdenum collection zone) provided with spaced electrodes through which a direct electric current is passed in order to collect a significant portion of the residually dissolved molybdenum in solid form at the negative electrode (cathode) of the electrolytic cell.
2. Prior Art
It is known to react an olefinically unsaturated compound such as propylene with an organic hydroperoxide such as tertiary butyl hydroperoxide in the presence of a soluble molybdenum catalyst to provide a reaction product comprising propylene oxide and tertiary butyl alcohol. See, for example, Kollar U.S. Pat. No. 3,350,422, Kollar U.S. Pat. No. 3,351,635, and Russell U.S. Pat. No. 3,418,340.
It is also known to prepare soluble molybdenum catalysts to catalyze the reaction as disclosed, for example, in Bonetti et al. U.S. Pat. No. 3,480,563, Shum et al. U.S. Pat. No. 4,607,113, Marquis et al. U.S. Pat. No. 4,626,596, Marquis et al. U.S. Pat. No. 4,650,886, Marquis et al. U.S. Pat. No. 4,703,027, etc.
Kollar U.S. Pat. No. 3,860,662 is directed to an improvement in his basic process relating to the recovery of alcohols from the reaction product, which product is stated to be of an acidic nature, wherein a basic material such as an alkali metal or alkaline earth metal compound is added to the reaction mixture. Kollar U.S. Pat. No. 3,947,500 discloses a method for treating the reaction product formed by the reaction of an organic hydroperoxide with an olefin wherein an organic alcohol is formed as a by-product. It is stated that the alcohol tends to dehydrate and that to at least partially overcome this problem the oxidation reaction product is treated with an alkali metal or an alkaline earth metal compound. Kollar states that the alkali metal or alkaline earth metal compound can be added to the epoxidation reactor or to the reaction product.
Sorgenti U.S. Pat. No. 3,573,226 discloses a method wherein a molybdenum-containing catalyst solution is prepared by incorporating metallic molybdenum into the distillate bottoms fraction of an epoxidation reaction product followed by heating of the resultant mixture in order to form a soluble molybdenum-containing reaction product which can be used to catalyze the epoxidation reaction.
Maurin U.S. Pat. No. 3,931,044 is directed to a method for recovering molybdenum catalyst values from a epoxidation reaction product for recycle. Maurin discloses one of three techniques. In accordance with the first embodiment, the residue fraction is calcined to provide molybdenum trioxide which is then used to prepare a soluble molybdenum compound by reaction with aqueous ammonia. In a second embodiment, the molybdenum-containing fraction is treated with aqueous ammonia without calcining to form an ammonium molybdate which is treated with a polyalcohol to give a molybdic ester. In a third embodiment, the molybdenum-containing fraction is treated with gaseous ammonia in order to form an ammonium molybdate precipitate which can be recovered by filtration.
Harvey U.S. Pat. No. 3,449,217 is directed to a process for the recovery of tertiary butyl hydroperoxide from a mixture comprising tertiary butyl hydroperoxide, tertiary butyl alcohol and organic acids and esters resulting from the liquid phase oxidation of isobutane by a process which minimizes hydroperoxide decomposition. This is done by accomplishing the distillation while the product has an effective pH of below about 9. The patentee teaches the treatment of the reactor effluent with a neutralizing agent such as an alkali metal or an alkaline earth metal hydroxide.
Levine U.S. Pat. No. 3,819,663 is directed to a method for treating a heavy distillation fraction of this nature in order to recover the molybdenum in the concentrated bottoms fraction for recycle to the epoxidation reaction zone as makeup catalyst.
Levine conducts his wiped-film evaporation process under conditions including a temperature of about 550.degree.-650.degree. F. (about 273.degree. to about 330.degree. C.) at atmospheric pressure to obtain his desired residual fraction for recycle as catalyst makeup and a distillate fraction comprising about 85% or more of the heavy distillation fraction. Levine states that the distillate fraction that is thus obtained can be used as a furnace fuel or can be worked up for recovery of the individual components contained therein. However, Levine et al. does not contain any teaching as to how the individual components in the fraction would be obtained.